Digital lighting technologies, i.e. illumination based on semiconductor light sources, such as light-emitting diodes (LEDs), offer a viable alternative to traditional fluorescent, HID, and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, durability, lower operating costs, and many others. Recent advances in LED technology have provided efficient and robust full-spectrum lighting sources that enable a variety of lighting effects in many applications. Some of the fixtures embodying these sources feature a lighting module, including one or more LEDs capable of producing different colors, e.g. red, green, and blue, as well as a processor for independently controlling the output of the LEDs in order to generate a variety of colors and color-changing lighting effects, for example, as discussed in detail in U.S. Pat. Nos. 6,016,038 and 6,211,626.
It is well-known that by mixing light with different spectra such as red, green, and blue light, it is possible to generate light of different colors. Accordingly, varying the intensity of radiation from different color LEDs, such as the commonly available red, green, and blue LEDs and optionally amber LEDs, can give the perception of an output light of any desired color, including white light.
Aspects of the resultant output light, such as chromaticity, are dependent on the combination of the intensities and center wavelengths of the LEDs combined to produce the output light. These optical parameters can fluctuate even when the LED drive current is constant, due to such factors as heat sink thermal constants, changes in ambient temperature and device aging.
One approach to alleviate this problem is to employ optical feedback to continuously monitor the radiant flux output of the different color LEDs so as to adjust the drive currents of the LEDs such that the luminous flux and chromaticity of the output light remain substantially constant. This monitoring requires some means of measuring the radiant flux output of each LED color.
To date, several optical feedback solutions have been proposed to detect and evaluate the luminous flux and chromaticity of the output light in order to provide for correction if these values deviate from a desired color point. For instance, a number of approaches rely on an array of photosensors, each having a selected color filter responsive to light of a selected color. However, these photosensors are prone to optical crosstalk and suffer from inaccuracies in the measurement of the characteristics of the output light due to the overlap in the spectral radiant power distribution of the light emitted by LEDs of different colors.
A partial solution to this crosstalk problem is to select bandpass filters with narrow bandwidths and steep cutoff characteristics. Although satisfactory performance levels for such filters can be achieved using multilayer interference filters, these filters can be expensive and typically require further optics for collimating the output light, as the bandpass wavelengths are dependent on the incidence angle of the output light upon the filters.
Another problem associated with interference filters is that the center wavelengths of high-flux LEDs are dependent on the LED junction temperature. In addition, the bandpass transmittance spectra of interference filters are also temperature dependent. The output signal of the photosensor is dependent on the convolution of the spectral radiant power distribution of the LED and the bandpass characteristics of the filter. Therefore, the output signal of the photosensor may change with ambient temperature even if the LED spectral radiant power distribution remains constant, which can further limit the performance of an optical feedback system.
In another approach, each LED in a multi-color LED-based lighting system is controlled by an electronic control circuit, which selectively turns OFF the LEDs for the colors not being measured in a sequence of time pulses using a single broadband optical sensor. The average light output during the measuring period can be substantially equal to the nominal continuous light output during the ordinary operation to avoid visible flicker. A difficulty associated with this approach is that color balance is periodically and potentially drastically altered each time the LEDs are de-energized, causing noticeable flicker. Since the optical sensor requires a finite amount of time to measure the radiant flux of the energized LEDs with sufficient accuracy and acceptable signal-to-noise ratio, the sampling frequency can be limited by the response time of the optical sensor. A limited sampling frequency can result in lower sampling resolution and longer response times for the optical feedback loop. Moreover, since the LED colors are to be measured sequentially, this approach for optical data collection can further increase the feedback loop response time by a factor of three for a system with red, green, and blue LED clusters and a factor of four for a system with red, green, blue, and amber LED clusters.
A similar approach seeks to alleviate the flicker by selectively measuring the light output of the LEDs in a sequence of time pulses, whereby the current for the color being measured is turned OFF. Neither of these proposed solutions however, addresses the periodic and potentially drastic changes in the color balance or the reduction in feedback loop response time.
In yet another approach, the light output of the LEDs is sampled by a broadband optical sensor during the duration of the PWM drive pulse where the pulse has reached full magnitude, so as to avoid the effect of the rise and fall times of the PWM pulse. The average drive current is then determined by low pass filtering. A difficulty associated with this approach can be that the PWM pulses must be synchronized such that at least one LED color is de-energized for a finite period of time during the PWM period. This requirement can prevent operation of all different color LEDs at full power at 100% duty factor. Another disadvantage associated with this average light sensing is that the sampling period must provide sufficient time for the optical sensor to reliably measure the radiant flux of the energized LEDs, in addition to a requirement that the LED colors must be measured sequentially, which can limit the feedback loop response time.
Another approach is to provide an apparatus for controlling a light source wherein the light source includes at least one light source that emits light with a superimposed optical signal at a discrete frequency and an electronic reference signal at a discrete frequency. The apparatus includes a photodetector optically coupled to the light source and designed to receive the light signal. The apparatus includes at least one lock-in system coupled to the photodetector and each light source that receives the light signal from the photodetector and receives the reference signal from the light source. Each lock-in system produces an intensity value of the light source based on the light signal and the reference signal. The lock-in system may include a signal multiplier and a filter coupled to the signal multiplier wherein the intensity value is the product of the light signal and the reference signal processed through the signal multiplier, and filtered to remove non-DC portions. While this apparatus can provide for the detection of light contribution, there can be an inherent error that enters this format of a system, thereby limiting the effectiveness thereof for control of light output by the apparatus. Furthermore, this apparatus does not provide for driving LEDs using sophisticated drive techniques, such as pulse-width modulation with a controllable duty cycle.
Thus, there is a need in the art for a new optical feedback method and apparatus that can provide radiant flux output data for a plurality of light sources in a mixed light system using a broadband optical sensor.